Infrared chalcogenide glass

ABSTRACT

An infrared-transmitting glass material consists essentially of 35.3% wt. arsenic and 64.3% wt. selenium and has an expansion coefficient of 27×10 −6 /° C.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a chalcogenide glass material used asoptical fibers capable of transmitting infrared radiation, and moreparticularly to a unique composition of arsenic and selenium.

BACKGROUND OF THE INVENTION

Glasses based on the elements sulfur, selenium and tellurium have beenformulated and produced as infrared optical materials for many years.The emphasis has usually been to identify a composition with goodoptical properties and with favorable physical properties as well.Desirable features of such glasses include strength, hardness, highsoftening points, low thermal expansion and resistance to thermal shock.The desire is to have sturdy optics for application in systems to beused in the field for example by the military.

However, different applications require glasses with differentproperties. For example, glasses to be drawn into fibers should not bebrittle. Experience has shown that the strongest high softening glassesdo not make the best fiber, with fused pure silica being an exception.It is desirable to have a lower softening glass because the drawingequipment is easier to fabricate and has a longer life. The same may besaid about molding or extruding glass. The processes carried out atlower temperatures are easier and less expensive. The molds or extrudershave a longer life operating at lower temperature. Of course the glassmust have a softening point that meets the requirement of theapplication.

The success of the pure fused silica fiber is well known. The successfor infrared transmitting chalcogenide glass fibers is not nearly asgood. Chalcogenide glasses are much weaker. Drawn fibers are only afraction as strong, about one seventh, in comparison to silica fibers.Infrared optical absorption is orders of magnitude greater in comparisonto silicates. There are a number of good infrared lasers that emit wattsof energy in the infrared. It would be desirable to have a flexibleinfrared transmitting glass fiber capable of transmitting that energyfrom the laser to an inaccessible location such as in the case of usefor surgery. The carbon dioxide laser which emits intense infraredradiation continuously at 10.6 micrometers, has been approved forsurgical use by the FDA. However, suitable flexible glass fiber suitablefor this purpose has not been found.

Because the thermal change in refractive index for the glass used tomake the fiber has an appreciable positive magnitude, as the energyflows through the fiber, the phenomena “thermal lensing” occurs. Thermallensing may be best described as self focusing within a solid broughtabout by a radial change in refractive index about the center ray of atransmitted beam due to absorption, generated heat and the thermalchange in refractive index for the material. Glasses are particularlysusceptible because their disordered structure leads to low thermalconductivity. The absorbed radiation heats up the fiber in a non uniformmanner producing a lensing action in the fiber which focuses the laserenergy burning the fiber into. The only exception to this occurrence wasfiber made from arsenic trisulfide glass. It has been reported that over70 watts of laser power from a carbon monoxide laser emittingcontinuously at 5.4 Micrometers was transmitted through an arsenictrisulfide fiber 400 micrometers in diameter without failure. Thethermal change in refractive index for the glass at that wavelength waszero, so no thermal lensing occurred.

A need has arisen for a composition that has a zero thermal change inrefractive index resulting in a glass suitable for molding which issoft, has a low softening point which would result in a very largethermal expansion.

SUMMARY OF THE INVENTION

An infrared-transmitting glass material consists essentially of 35.3%wt. arsenic and 64.3% wt. selenium and has an expansion coefficient of27×10⁻⁶/° C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention comprises an arsenic-selenium glass composition inthe proportion of about 35.3% wt. percent arsenic and: 64.7% wt.selenium. Variations in these proportions are approximately +/−2% wt.The expansion coefficient is 27×10⁻⁶/° C. which is approximately thevalue of the expansion coefficient of aluminum. Thus, the glass can beattached to mountings, mounting rings or flat surfaces made fromaluminum without stress or fracture due to temperature excursions. Thethermal change in refractive index is less than 1×10/° C. at 4micrometers and 10 micrometers wavelength. The present glass compositionallows infrared transmitting glass fibers to transmit large amounts ofcarbon dioxide laser CW power without burning, and without thermallensing.

Other alteration and modification of the invention will likewise becomeapparent to those of ordinary skill in the art upon reading the presentdisclosure, and it is intended that the scope of the invention disclosedherein be limited only by the broadest interpretation of the appendedclaims to which the inventor is legally entitled.

1. An optics system comprising: an optical element including achalcogenide glass composition having a proportion of arsenic in therange of 37.3% wt. to 33.3% wt. and a proportion of selenium in therange of 62.7% wt. and 66.7% wt. and having a thermal expansioncoefficient of about 23.6×10⁻⁶/° C. and a thermal change in refractiveindex less than about 1×10⁻⁶/° C.; and a mounting device for saidoptical element including aluminum material.
 2. A fiber optic systemcomprising: glass fibers including a chalcogenide glass compositionhaving a proportion of arsenic in the range of 37.3% wt. to 33.3% wt.and a proportion of selenium in the range of 62.7% wt. and 66.7% wt. andhaving a thermal expansion coefficient of about 23.6×10⁻⁶/° C. and athermal change in refractive index less than about 1×10⁻⁶/° C.; and amounting device for said fibers including aluminum material.